Minute to small (0.5-3 mm), rarely larger (up to 8 mm, wing length up to 15 mm), delicate Nematocera. Eyes holoptic with the exception of a few genera with reduced wings; mouthparts reduced; antenna in general conspicuously long, usually with 12-14 segments, sometimes less, sometimes more, up to 40; antennal segments bearing differently-shaped sensoria, in some Porricondylinae and in all Cecidomyiinae these are thread-like, in the supertribe Cecidomyiidi the sensoria (some or most) in the shape of long loops; ocelli present (in the Lestremiinae) or absent. Wing usually clear, in a few species with a pattern; number of longitudinal veins reduced; costa usually with a break just beyond vein R5. Legs long, tibiae lacking apical bristles.

The larvae and adults can be found in a large array of habitats. Three main groups can be distinguished with respect to the feeding mode of the larvae: 1. larvae living in, and feeding on, mushrooms, bracket fungi, dying and decaying wood, or other organic substrates (soil, litter, plant remains, etc.); 2. larvae that cause damage to higher plants, either living free, in galls, or as leaf-miner; 3. larvae preying on invertebrates, in particular on (larvae of) small insects. Of the three subfamilies, Lestremiinae, Porricondylinae and Cecidomyiinae, only representatives of the last cause the formation of galls. Some species are of economic importance, because they cause damage to plants in agriculture and horticulture and others because of their use as biological control agents.